why can't we test the logged messages now ? can an equivalent log message be added in the relevant place in the new module ? that allows testing the real codepath (or something closer to it), instead of stubs.

The getTestLogger().getLoggedEntries() call was failing to pick up the logger. The issue with only testing a log line is that the test won't fail if the functionality is removed or changes (the login call with AWS ID is what we care about asserting here, not a logged string). I did not spend time to dive deeper into it because it's a secondary assertion IMO

The issue with only testing a log line is that the test won't fail if the functionality is removed or changes (the login call with AWS ID is what we care about asserting here, not a logged string)

Sure, but that applies to any kind of telemetry (aka logs): the telemetry is wrong if logic changes without updating the telemetry. And a similar situation applies when stubs/mocks are used: asserting that a method was called doesn't actually test that the functionality works, and can easily drift (also, asserting stubs leads to brittle tests).